I BODY-CAVITY II 
(e.g. Mysis) the first maxillipede is a typical biramous limb, 
though the expanded gnathobases in some forms are beginning 
to project (Fig. 1, E), while the limb following, which corresponds 
to the second maxillipede of Decapods, is simply a biramous 
swimining leg. Besides this obvious conversion of a biramous 
into a foliaceous limb, further evidence of the fundamental 
character of the biramous type is found, first, in its invariable 
occurrence in the Nauplius stage, which does not necessarily 
mean that the ancestors of the Crustacea possessed this type 
of limb in the adult, but which does imply that this type of 
limb was possessed at some period of life by the common 
ancestral Crustacean ; and, second, the limbs of the Trilobita, 
a group which probably stands near the origin of the Crustacea, 
have been shown by Beecher to conform to the biramous 
type (Fig. 1, H). Furthermore, the thoracic limbs of Mebalia, 
an animal which combines many of the characteristics of 
Entomostraca and Malacostraca, and is therefore considered as 
a primitive type, despite their flattened character, are really built 
upon a biramous plan (Fig. 1, G). 
In conclusion, we may point out that this view of the 
Crustacean limb, as essentially a biramous structure, agrees with 
the conclusion derived from our consideration of the segmenta- 
tion of the body, and points less to the Branchiopoda as 
primitive Crustacea and more to some generalised Malacostracan 
type. 
So far we have shortly dealt with those systems of organs 
which are clearly affected by the metameric segmentation of the 
body ; we must now expose the condition of the body-cavity to 
a similar scrutiny. If we remove the external integument of a 
Crustacean, we find that the internal organs do not lie in a 
spacious and discrete body-cavity, as is the case in the Annelids 
and Vertebrates, but that they are packed together in an irregular 
system of spaces (“haemocoel”) in communication with the 
vascular system and containing blood. In the Entomostraca and 
smaller forms generally, a definite vascular system hardly exists, 
though a central heart and artery may serve to propel the blood 
through the irregular lacunae of the body-cavity; but in the 
larger Malacostraca a complicated system of arteries may be 
present which pour the blood into fairly definitely arranged 
spaces surrounding the chief organs. These spaces return the 
